1. Field of the Invention
The present invention relates to a catalyst for and a method of purifying an exhaust gas containing an excess amount of oxygen, to remove nitrogen oxides (NO.sub.x), carbon monoxide and hydrocarbons therefrom. The present invention is particularly applied to the purification of exhaust gases originated from internal combustion engines of automobiles and nitric acid production plants.
2. Description of the Related Art
To purify an exhaust gas emitted from the internal combustion engine of automobiles and other engines, to thereby reduce toxic substances such as nitrogen oxides (NO.sub.x), carbon monoxide and hydrocarbons from the exhaust gas, a three-way catalyst which consists of platinum (Pt), rhodium (Rh), palladium (Pb) or other noble metals supported on a carrier material such as alumina (Al.sub.2 O.sub.3) is used. Nevertheless, these catalysts on a carrier cannot be advantageously applied to the purification of exhaust gas from diesel engines, because the exhaust gas contains an excess amount of oxygen which can adversely affect the selective reduction of the nitrogen oxides. Note, the term "oxygen-excess exhaust gas" or "exhaust gas containing an excess amount of oxygen" is intended to mean that the exhaust gas contains an excess amount of oxygen beyond the amount necessary to completely oxidize carbon monoxide, hydrocarbons and hydrogen in the exhaust gas.
Similar problems can be found in gasoline engines. Namely, to reduce fuel consumption and lower an amount of carbon dioxides in the exhaust gas, gasoline engines are necessary to operate under lean burn conditions, i.e., at a lean air-fuel ratio (A/F) higher than a theoretical air-fuel ratio. Nevertheless, since the exhaust gas produced in the gasoline engines apparently contains an excess amount of oxygen, the above-described three-way catalyst on a carrier cannot be applied to the purification of the described exhaust gas on the same grounds, although it can reduce only the carbon monoxide and hydrocarbons through an oxidation thereof, and a method of completely reducing toxic substances from the exhaust gas of engines such as diesel engines and lean burn gasoline engines has not been put to practical use to date.
As an alternative to selectively reduce nitrogen oxides (NO.sub.x) from the oxygen-excess exhaust gas, it is well-known to add a reducing agent such as ammonia to the exhaust gas, or to use an alkali to absorb the nitrogen oxides thereon. These well-known methods, however, do not ensure satisfactory results when applied to the purification of the exhaust gas from automobiles, because the automobiles are not a stationary object to which a purification unit can be easily mounted, and the exhaust gas is produced only while the automobiles are running. Namely, these methods have only limited fields of application.
In addition to the above-described purification methods, it is also well-known to use a catalyst having a transition metal introduced through an ion exchange on a zeolite substrate. For example, Japanese Unexamined Patent Publication (Kokai) No. 130735/1989 teaches a copper (Cu)-containing zeolite catalyst wherein Cu is introduced into the zeolite structure through an ion exchange. The described catalyst, when used to selectively reduce nitrogen oxides (NO.sub.x) from an oxygen-excess atmosphere, which also contains unburned carbon monoxide and hydrocarbons, can effectively reduce the nitrogen oxides to nitrogen (N.sub.2), a nontoxic substance. This reduction of the nitrogen oxides is considered possible because NO.sub.x is adsorbed on Cu, and then the adsorbed NO.sub.x is catalytically reacted with reducible, unburned hydrocarbons to thereby convert NO.sub.x to N.sub.2. The Cu-containing catalyst can exhibit a satisfactory NO.sub.x reducing capability at a temperature of 200.degree. C. or more, but suffers from the following drawbacks:
An initial catalytic activity of the Cu-containing zeolite catalyst is very high, because of good adsorption capability of Cu for NO.sub.x, but a durability of the catalyst, especially at an elevated temperature is poor. Namely, the high catalytic activity of the catalyst is notably deteriorates when the catalyst is exposed to a high temperature condition for a long time. Therefore, there is a need to provide an improved catalyst with an excellent catalytic activity which is not deteriorated when used for a long time. Note, it is considered that the Cu-containing zeolite catalyst has such an insufficient durability only because Cu can migrate and aggregate in the zeolite at a temperature of about 600.degree. C. or more, to thereby cause a loss of the catalytic activity of the catalyst, and a stability of the catalyst structure is lowered as a result of the presence of the contained Cu, and accordingly, the catalyst structure itself is destroyed after a long term use of the catalyst. PA1 M is a cation, PA1 n is a valency of the cation M, PA1 x is 0.8 to 2, PA1 y is at least 2, and PA1 z is at least zero. The catalyst comprises a zeolite having a molar ratio of SiO.sub.2 to Al.sub.2 O.sub.3 of at least 15, and containing cobalt alone or in combination with at least one metal selected from alkali earth metals and rare earth metals.